The techniques of magnetic susceptibility and rapid reaction kinetics will be used to study the structure, function and dynamics of the active sites of metalloproteins. To accomplish this we have developed a unique high-sensitivity time-resolved magnetic susceptometer. This instrument, which is based on recent advances in superconducting technology, is 100 times more sensitive than conventional susceptometers and provides microsecond time resolution for studying the rapid kinetics of changes in oxidation or spin state of the active site metal ions as the protein functions. A number of the proposed studies are directed at a detailed understanding of the mechanism of cooperative oxygen binding to hemoglobin. Such an understanding is relevant to both the physiology of O2 transport, and a general understanding of allosteric enzymes and protein control of metal ion activity. The dominant proposals for the hemoglobin cooperative mechanism invoke coupling between the spin state of the heme iron and changes in protein structure. Our recent magnetic studies of human, mutant, and carp ferric (met) hemoglobins suggest that this is incorrect. We propose further critical tests of the Perutz Stereochemical model of cooperativity, using Time Resolved Magnetic Susceptibility (TREMS) and optical kinetic studies of valence hybrid hemoglobins to further quantitate the linkage between spin states and protein structure. We also propose studies of the kinetics of changes in quaternary structure in hemoglobin, and tests of recent proposals that there are intermediate quaternary structures and that transitions between them are governed by the distribution as well as the number of O2 bound per hemoglobin tetramer. We are also proposing the first TREMS investigations of some enzymes involved in important oxidation-reduction reactions, and which contain several redox-active metal centers. It is hoped that these measurements will help to delineate the electronic structure of reaction intermediates, and possibly also the dynamics of electron flow between metal sites. Such information should contribute to a better understanding of how these enzymes are able to store redox energy from single electron transfers in order to carry out multi-electron oxidations or reductions. Other magnetic studies of heme, copper, and iron-sulfur proteins are proposed to answer specific questions about their electronic configurations and active site structures.